Canister-type drain valves can have poor flush stability, easy to leak when used to implement a double-flush drainage function. In some instances, water for large-volume and small-volume flushing can be flushed through two vertically disposed barrels, an upper barrel being a buoy, and a bottom buoy being a buoy during no flushing and large-volume flushing, and injected with water during small-volume flushing.